Software attack tools that turn PCs into remotely controlled zombies are getting better, but defenses are not keeping up, say security experts. The latest threats are tailored to attack specific companies, foregoing rapid spread and avoiding notice. Others use modular components, such as an infector that can be changed to defeat the latest antivirus software and a second-stage component that turns off PC defenses.

"While many of these features have appeared in one virus or another, the level of sophistication of the latest malware is what's interesting," said Roger Thompson, director of malicious content research for information-technology giant Computer Associates. "It definitely can cause problems for current defenses."

In early June, Thompson and other antivirus researchers warned that the latest bot software--a program designed to compromise and control other computer systems--spread in multiple stages. The warnings followed revelations that several companies in Israel had reportedly used a previously unknown Trojan horse program to infiltrate competitor's systems. The Trojan horse reportedly used by the companies, dubbed Hotword, was not detected because it had not been deployed elsewhere.

Such attacks highlight the deficiencies of current defenses that detect known threats using pattern--or signature--matching and heuristics, said Thompson.

"Any delay in detecting the threat is already too long," he said. "In my mind, this is the death knell of signature scanners."